#21385 acpmux: bind npm updates to the detected installation

An agent installed under another npm prefix could update the shell's default installation instead. Bind update argv to the detected absolute prefix and recorded package, including forks. Invalid or unbound saved plans lose their update command while retaining version observations.

Validation: formatting, clippy, compilation, 74 harness tests and 17 health tests pass on the reviewed fleet. Regressions reproduce the wrong-target and stale-cache behavior before the fix. Stand-in package managers cover spaces, quotes, Unicode and distinct installations.

Stacked on #21322.

Changelog

Fixed: npm agent updates target the detected installation and preserve its package or fork.